Supernovas themselves are practically Creation in its purest form. Only in a Supernova is it possible for the heavier elements to form. In other words: Without a Supernova there would be no Earthlike planets, no DNA, no life.
(And technically this of course also means that the atoms that constitute our bodies were sometime long long ago all part of an exploding Star, which is kinda cool
I.e. We're made of "Star Stuff" as Carl Sagan so famously said.)
So you actually could say the Shivans not only preserve the younger (non-spacefaring races) and destroy the older (subspace using) races, but they also took part in the potential Creation of new life itself.
If that was their actual intent however, is anyones guess LOL
The mere timescale involved would be practically impossible to grasp for the human intellect. (I.e. today i'm exploding this star, so a couple of billion years after i'm dead there may be new life heh.)